This invention relates to the method of using an earth sensor on a satellite or spacecraft for providing three axes attitude information for the orbiting satellite or spacecraft using the earth and another celestial body.
Unmanned earth orbiting satellites generally employ earth sensors which are mounted therein and used for attitude control and guidance of the orbiting satellite. The attitude of the satellite is determined by its position with respect to three axes at right angles to each other. Two of these axes are in a plane at right angles to a projected radius of the earth passing through the satellite and the third axis coincides with such radius. The plane in which the first two axes lie is parallel to the earth's horizon and once the orientation of the satellite with respect to these two axes is determined, orientation around the other axis can be determined using other means such as dedicated sun-sensors, star-trackers, magnetometers, or other such equipment for observations of other heavenly bodies, or physical phenomena.
Earth sensors utilize a line of thermal discontinuity between the earth's atmosphere and outer space, which is characterized by a large differential in infrared radiation between outer space and the earth which is considerably warmer. This characteristic is utilized in one embodiment on the invention by scanning an infrared detector in a conical scan pattern across the horizon and deriving electrical output signals marking the line of thermal discontinuity. Reference pulses are also generated as the scan passes through predetermined points in the vehicle. The intervals between crossings and the reference pulses are compared to produce an output signal, which produces information with respect to the attitude of the vehicle with respect to the earth which it orbits.
Earth sensors generally treat the sun or other celestial bodies as unwelcome intrusions into the process of measuring spacecraft attitude. Generally, the sun is identified by its small angular size and its high intensity, which is rejected. Some horizon sensors actually provide sun sensors for detecting the sun in order to eliminate the sun or other celestial bodies from the attitude measuring process.
Other celestial bodies may be used for providing attitude information but again may require additional sensors, which are specifically directed at the celestial body to be utilized. One of the problems in using a single sensor is that the angular subtense of such celestial objects may be so small that in using a fixed scan circle, the celestial body may be missed on any given scan. Unless the object is intercepted repetitively by a particular scan pattern, the accuracy and/or reliability of the attitude measurement cannot be maintained.